Fuel injection systems deliver fuel to the combustion chamber of a gas turbine engine, where the fuel is mixed with air before combustion. One form of fuel injection system well-known in the art utilises fuel spray nozzles. These atomise the fuel to ensure its rapid evaporation and burning when mixed with air.
An airblast atomiser nozzle is a type of fuel spray nozzle in which fuel delivered to the combustion chamber by a fuel injector is aerated by air swirlers to ensure rapid mixing of fuel and air, and to create a finely atomised fuel spray. The swirlers impart a swirling motion to the air passing therethrough, so as to create a high level of shear and hence acceleration of the low velocity fuel film.
Typically, an airblast atomiser nozzle will have a number of coaxial air swirler passages. An annular fuel passage between a pair of air swirler passages feeds fuel onto a prefilming lip, whereby a sheet of fuel develops on the prefilming lip. The sheet breaks down into ligaments which are then broken up into droplets within the shear layers of the surrounding highly swirling air to form the fuel spray stream that enters the combustor.
A conventional airblast fuel injector for a fuel spray nozzle has, in order from radially inner to outer, a coaxial arrangement of an inner air swirler passage, an annular fuel passage, an annular outer air swirler passage, and an annular shroud air swirler passage. Mixing of air flow from all three air swirler passages is desirable to minimise smoke and emissions. The outer and shroud air passages have convergent portions which direct their swirling air flows radially inwards, creating shear layers between the air flows and promoting turbulent mixing.
The convergent portion of the outer air passage and the convergent portion of the shroud air passage are typically divided by an annular wall. If the shroud air flow separates from the wall, combustion can occur in this region, producing high metal temperatures which can result in metal loss and consequent deterioration of component performance.
Accordingly, it is desirable to provide a fuel injector that is less susceptible to high metal temperatures.